Clandestine nuclear weapons are an immediate threat to every country and every city in the world. A rogue nation with a nuclear weapon, or a terrorist group that acquires radiological material, could deliver it to a victim city via commercial shipping at low cost and low risk. Nuclear weapons are difficult to detect when shielded. Advanced radiation detectors are necessary to reveal such weapons among backgrounds and benign clutter. An urgent priority of the United States, and indeed of all countries, is the development of radiation detectors that both detect and localize clandestine nuclear material.
A signature of all nuclear and radiological weapons is radiation, principally gamma rays (“gammas”) and neutrons. Gamma rays are detected when they interact with matter via photoelectric absorption in which the gamma ray is absorbed and a photoelectron is emitted, Compton scattering which generates a Compton electron and a scattered gamma ray, or electron-positron pair production. In each case, the energetic electron (or positron, treated as an electron herein) can be detected in a charged-particle detector such as a scintillator. Neutrons are usually classified according to energy as fast, intermediate, and slow. A fast or high-energy neutron, as used herein, has 100 keV to several MeV of energy. Fast neutrons can be detected by neutron-proton elastic scattering in which the recoil proton passes through a detector such as a scintillator. Slow or low-energy neutrons (1 eV or less, also called thermal or epithermal) are detected by a capture reaction in a neutron-capture nuclide, usually 10B or 6Li, causing emission of prompt ions such as alpha and triton particles which can be detected in a scintillator or other ionization detector. Intermediate-energy neutrons may be moderated or decelerated by multiple elastic scattering in a hydrogenous material such as HDPE (high-density polyethylene), and then detected as slow neutrons.
Numerous directional radiation detectors have been proposed. Typically, they have one-dimensional directionality, meaning that they indicate whether the source is to the left or right of the detector. Multiple iterative rotations are then needed to specify the source location in one dimension, such as the bearing of the source in a horizontal plane. This iterative rotation process is extremely time-consuming. In addition, a one-dimensional scan is not sufficient to localize a threat in large inspection items such as trucks and railcars and shipping containers; a two-dimensional determination is needed. Although a pair of one-dimensional directional detectors might conceivably be used to separately scan horizontally and vertically, this would require two separate systems and would entail some kind of coordination between them. Also, the two systems would each have their own background rate, further diluting the threat signature and requiring longer scan times. Alternatively, a single prior-art directional detector might conceivably be able to scan horizontally first, then roll by 90 degrees, and then scan vertically, but this would take twice as long and would require a complicated mechanical joint.
Prior art further includes ostensibly directional gamma ray detectors (U.S. Pat. No. 8,319,188 to Ramsden, U.S. Pat. No. 7,944,482 to Frank, for example) comprising four scintillators packed around a detector axis and analyzed to determine an azimuthal angle and, in the case of the Ramsden device, a low-resolution indication of the polar angle as well. These configurations lack a central shield and thus must depend on the various scintillators to shield each other, which greatly limits the angular contrast achievable.
What is needed, then, is a gamma ray or neutron detector system with two-dimensional directionality, that preferably provides superior angular resolution extending all the way around the detector including polar angles near the midplane of the detector, and with enough sensitivity to detect a shielded source. Preferably the detector would indicate the direction toward the source, in two dimensions, using a single data set acquired at a single orientation of the detector, thereby avoiding the need for iterative rotations. Preferably such a detector would be compact, fast, efficient, easy to build, easy to use, and low in cost.